Threaded fasteners, such as bolts and screws, have been used in a variety of application for many years. Conventional bolts include a thread that is a continuous helical ridge formed on the outside of a cylindrical body. The topmost point on this ridge is called the crest. Between each crest is a space and the bottommost point in this space is called the root. In common bolts, threads are set at an angle to the axis of the bolt, which is called the helix angle. The angle must be sloped, either upward to the right for right-hand threaded screws or upward to the left for left-hand threaded screws. Thread pitch is the distance from the crest of one thread to another crest measured along the length of the thread. The lead distance is the width across the crests of a single or multiple threads.
Conventional threads are designated or named by the external thread major diameter and a pitch measurement. The major diameter is the outer diameter at the top of the thread crests. Thread sizes are given in nominal sizes, not in the actual measurement, and the exact measurement is slightly below the named or nominal size.
Threads are sometimes identified as “fine” or “coarse”. A fine thread will have a relatively small pitch measurement, and the threads will be closer together. A coarse thread has a relatively larger pitch measurement, and the threads will be further apart. A fine thread will have less depth as compared to a coarse thread, and consequently are easier to strip. A coarse thread is more resistant to stripping but also less efficient in transmitting torque into thread tension. Generally, a fine pitch is easier to tighten in that tension is achieved at lower torques.
Conventional threaded fasteners are widely used and are generally effective. However, conventional threaded fasteners have a number of drawbacks that make them inappropriate in certain applications. For threads to interchange and match, both the diameter and pitch must match. Even when threads are properly sized, there will be play or slop between external and internal threads when engaged. This play is thought to be normal and is supposed to disappear when the fastener is tightened, allowing a thread to be a bit larger or smaller than ideal while allowing the bolt to still function adequately. However, if tolerances are exceeded, the fastener may require excessive force to install, causing the thread to fail during tightening, or may result in a sloppy fit, compromising the holding power of the fastener.
Tightened threads subjected to vibration loosen because of the clearance space. This clearance is needed to engage the threads, such as in a nut and bolt, with a reasonable amount of friction. Its drawback is that it becomes the ramp that the screw or nut will follow and loosen subject to thermal changes and/or vibration.
Further, because of the unreliable fit of conventional threaded fasteners, and their substantially constant diameter, sealing compounds or gaskets are needed when such fasteners are used in connection with packaged foods or in pressurized applications, such as plumbing, in order to ensure that fluids do not migrate through gaps in the threads and cause leaks. The use of such compounds or gaskets significantly adds to the cost of these applications. Further, even if compounds or gaskets are applied, it is not readily apparent whether such compounds or gaskets are providing an adequate seal.
Another, disadvantage of conventional threaded fasteners is that it is difficult to determine their depth, which allows them to be over tightened to extend beyond their intended length. This disadvantage is of greatest detriment in the case of screws, where screw tips can extend beyond the backside of the material to be joined, but is likewise an issue with conventional bolts.
Still another disadvantage of conventional threaded fasteners is the need to properly center the fastener within the hole. Improper centering can result in stripping of the threads and, therefore, care must be taken to ensure that the threads are properly centered to mate with the opening.
Finally, because conventional threaded fasteners require a large number of threads to be engaged in order to provide adequate holding power, and because these threads are of a substantially fixed major diameter, conventional threads must be rotated a large number of times in order to adequately secure them. Accordingly, high-speed assembly of conventional threaded fasteners requires the use of an automated screw gun, or variable speed drill equipped with a driver bit. These devices are cumbersome, expensive, and pose a high risk of stripping the head of the bolt or screw.
In response to these problems, the inventor of the present invention developed the conic threaded fastener disclosed in co-pending patent application Ser. No. 11/178,890, filed on Jul. 11, 2005, which is incorporated herein by reference in its entirety. However, it has been found that the conic threaded fastener presents certain manufacturing challenges that have heretofore prevented it from gaining widespread acceptance. Accordingly, there is a need for a fastening system that has the advantages of the conic threaded fastener, but is substantially easier to manufacture.
Therefore, there is a need for a threaded fastener that produces a tight seal without the use of gaskets or sealants, does not have a clearance space, that is of fixed length and cannot extend beyond its intended design length, that may be tightened, either automatically or by hand, more quickly and easily than conventional fasteners, that is self-centering and therefore fast to setup, that eliminates the need for a screw gun, that distributes pressure on the threads over a larger surface area of the teeth than a cylinder shape, that may be quickly tightened, and that is substantially easier to manufacture than the inventor's conic threaded fastener.